This invention relates generally to air handling systems. More particularly the invention relates to devices that improve the flow characteristics of air in ducts and across heat exchangers.
In many applications involving the flow of air through ducting in air handling systems, air flows from an exhaust port in a first duct into another duct, plenum or other larger volume. As the air exits the first duct, the air stream undergoes spreading. The jet spreading angle from a simple exhaust port such as the end of a circular ventilation pipe tends to be on the order of 5 to 8 degrees half angle. This is shown schematically in FIG. 1 where air exiting the end of duct 10 spreads to half angle .alpha. as it enters the space downstream of the duct. The spreading half angle is a function of the eddy structure of the free shear layer turbulence at the exhaust port.
There are many applications in which a larger spreading angle would be highly desirable. One such application is the interface between the air flowing through a duct and a downstream heat exchanger. A heat exchanger is frequently installed in a plenum that is larger in cross sectional area than the ducting upstream of the heat exchanger. Heat exchange efficiency would be enhanced if there were a uniform amount of air flowing over and through the entire face of the heat exchanger. However, if the spreading angle of the air leaving the upstream duct is not sufficient, the bulk of the air may flow through just a portion of the face of the heat exchanger with other portions receiving insufficient air flow for optimum performance.
Other benefits accrue when a larger spreading angle can be achieved at an exhaust port. There is greater flow uniformity leading to reduced air flow noise and reduced pressure losses in the ducting downstream of the exhaust port.
Frequently, the configuration of an air handling system includes bluff bodies that are installed in the ducting. A typical example of such a bluff body is a motor installed to power a fan in the duct. FIG. 2A shows schematically bluff body 02 installed in air flow duct 01'. As air flows around the bluff body, the wake immediately downstream of the body forms into a recirculation zone where air swirls in an eddy and does not flow smoothly down the duct. The result is a velocity profile across the duct as is shown in FIG. 2B, where the is a pronounced decrement in the downstream flow velocity immediately behind the bluff body.
Prior art efforts to produce streamline turning or increase jet spreading angle have included turning vanes, screens, perforated plates and other flow deflectors or obstructions located downstream of an exhaust port or bluff body. Although such devices can improve downstream flow uniformity, they do so at the cost of excessive and undesirable pressure losses. Other prior art devices have included various convoluted surfaces to reduce bluff body and blunt trailing edge airfoil drag. These devices are generally effective but do not include ejectors and therefore do not offer the wake minimization and drag reduction potential that an ejector feature combined with a surface convolution can.